At present, an organic electroluminescent device is generally composed of an anode, an organic layer and a cathode, wherein, at least one layer of the organic layer has luminescence function, and generally the light emitting layer of an OLED device is required to emit lights with three wavelengths, i.e. a blue light (B), a green light (G) and a red light (R). Generally, the blue light emitting material may be fluorescence emitting material, TTA light emitting material, or phosphorescence emitting material; the green light emitting material is a phosphorescence emitting material; and the red light emitting material is a phosphorescence emitting material. The luminescence mechanism of fluorescence is the use of singlet excitons accounting for 25% of the total excitons generated after the carrier recombination; the luminescence mechanism of TTA is the generation of a singlet exciton from two triplet excitons, which uses 62.5% of the total excitons; and the luminescence mechanism of phosphorescence is the use of singlet excitons and triplet excitons generated after the carrier recombination.
When the blue light emitting material is a fluorescence emitting material, the dopant is mainly perylene derivatives, oxadiazole derivatives, and anthracene derivatives, singlet excitons accounting for 25% of the total excitons are utilized, and the maximum external quantum yield is not more than 5%, thereby causing too high power consumption and waste of energy. When the blue light emitting material is a TTA material, host material in the light emitting layer is mainly used to complete the triplet-triplet annihilation process for luminescence, 62.5% of the total excitons generated are utilized and such ratio is higher than that of the fluorescence, but theoretically 100% utilization of the generated excitons cannot be achieved, thereby causing waste of energy. When the blue light emitting material is a phosphorescence emitting material, the dopant is mainly organic complexes of heavy metals such as iridium, platinum, and ruthenium, theoretically 100% utilization of the generated excitons can be achieved, however, due to the longer lifetime of the triplet exciton, there will appear a state that the concentration of exciton is too high which will induce the quenching among the excitons, thereby causing energy inactivation and thus the lifetime of the device is shorter.
In 2011, Professor Adachi, et al. from Kyushu University, Japan, reported a thermally activated delayed fluorescent (TADF) material with good luminous performance. The energy gap between state S1 and state T1 of such material is smaller and the lifetime of the exciton in state T1 is longer. Under certain temperature conditions, the exciton in state T1 can realize the process of T1→S1 by reverse intersystem crossing (RISC), and then experience radiation attenuation from state S1 to ground state S0. Therefore, luminous efficiency of the OLED device using such material as the light emitting layer can be comparable to that of the phosphorescence emitting material, and with no need for a rare metal element, thereby reducing the material cost.
There is still a need in the art to develop a method for improving the efficiency of a blue light organic electroluminescent device by using a thermally activated delayed fluorescent (TADF) material, and to provide an efficient and stable method for making an organic electroluminescent device.